The present invention relates to the shaping of glass sheets, and particularly to glass sheets having a painted border portion around the perimeter of one of its major surfaces.
1. Field of the Invention
Glass sheets are curved to fit the overall shape of a vehicle in which they are mounted. In recent years, in an effort to reduce the overall weight of automobiles to promote improved efficiency in terms of mileage per gallon of fuel consumed, the stationary windows of the automobile have been bonded to the mounting frames by a technique known as flush glazing. A peripheral border of opaque paint is applied to hide the ugly portions of the frame to which the shaped window is mounted.
It is easier to apply a border of paint to the peripheral portion of a flat glass sheet than to a peripheral portion of a curved glass sheet. Consequently, it has become customary to bake a painted border onto the surface of the flat glass sheet to which it is applied while heating the glass sheet to a deformation temperature before a solid member shapes the softened sheet during further processing. However, the paint is quite likely to stick on contact with the cover of a press bending mold used to shape the glass sheet, when the solid member is disengaged from the painted border portion.
It is customary in the art of press bending glass sheets to suspend a flat glass sheet from tongs which grip the sheet at its upper edge portion. The tongs are mounted on an overhead carriage which moves through a heating furnace, thereby conveying a glass sheet through the furnace. The time it takes the glass to traverse the furnace is coordinated with the heat delivered within the furnace to insure that the glass sheet is heated to a temperature sufficient for shaping. The deformation temperature of glass normally ranges from about 1040.degree. F. (560.degree. C.) to about 1110.degree. F. (600.degree. C.). Where the glass is also to be tempered in addition to being shaped, it is desirable to obtain surface temperatures on the order of 1200.degree. F. (650.degree. C.) to about 1250.degree. F. (675.degree. C.) before the glass sheet is shaped and, immediately thereafter, cooled rapidly enough to impart a desired temper to the glass sheet.
It has been conventional in the past to press bend glass sheets to desired shapes by engaging the entire opposite major surfaces of the glass sheet between a pair of press bending molds. A desirable press bending mold is depicted in U.S. Pat. No. 3,367,764 to Samuel L. Seymour. Another conventional mold construction involves a pair of shaping frames of complementary shapes that engage the opposite glass sheet surfaces at their marginal portions as in U.S. Pat. No. 3,256,080 to Jean Vranken. Also, glass sheets have been press bent using a combination of a continuous male mold that engages one major surface over its entire area and a frame mold that engages the opposite major surface at its marginal portion only. Inherently, such molds would engage a painted border portion during a glass shaping operation.
When glass sheets having painted borders are heated to temperatures needed for tempering, the paint sticks to the surface of the mold or the cover for the mold used to impress the shape into the heat-softened glass sheet. Furthermore, when the press bending molds retract from one another to permit the shaped glass sheet to be removed from the press bending station, the sticking of the peripheral portion of the painted glass sheet to one mold facing that painted glass sheet surface causes the sheet to be pulled out of shape until the one mold separates completely from the painted surface of the glass sheet. When the other surface of the glass sheet is free of paint, the glass sheet and other mold separate much more readily from one another, thereby leaving only the painted glass surface engaging the one mold, and causing the glass sheet to pivot about an axis defined by the tong gripping points. It would be inconvenient to apply paint to both major surfaces of the glass in order to attempt to equalize the sticking effect between the opposite surfaces in an effort to prevent the glass sheet from being distorted from its desired shape as the press bending molds retract.
2. Description of Patents of Interest
U.S. Pat. No. 3,169,900 to John R. Ermlich discloses a technique for pressing glass sheets wherein an opposite pair of press bending molds engage the opposite surfaces of tong suspended glass sheets in such a manner that the marginal portion of the glass is exposed. Such exposure causes the marginal portion of the glass sheet to develop a desired compression stress. The glass treated in this patent is completely free of any paint.
U.S. Pat. No. 3,346,358 to Dean L. Thomas discloses a method of press bending vertically suspended glass sheets using a pair of press bending molds having fabric covers over concave and convex shaping surfaces. The lower edge of a mold having the concave shaping surface is offset upwardly from the lower edge of the mold having the convex shaping surface so that its fabric cover rubs against the major surface of the glass sheet rather than along the bottom corner line formed at the intersection of the major surface of the glass sheet that faces the concave mold and the bottom edge surface of the glass sheet. Therefore, the purpose of the patent of Thomas is to reduce wear of the mold cover due to rubbing the mold cover for the concave shaping mold relative to a sharp corner of a glass sheet.